Introduction to AMT

Overview

Natural-source Audio-frequency Magnetotellurics

(AMT) is an electromagnetic survey

technique that uses naturally-occurring

ionospheric currents and lightning

storms — passive energy sources — to

electrically map geologic structure to

depths of 500 meters or more.

Natural-source electromagnetic (EM)

signals are generated in the atmosphere

and magnetosphere. The time-varying

electric and magnetic fields induce

currents into the earth and oceans,

which produce magnetotelluric (MT)

signals, which are measured by AMT

and MT data acquisition systems.

Low-frequency magnetotelluric EM signals

(< 1 Hz) are generated by the interaction

between the earth’s magnetosphere and

solar wind. High-frequency sources in

the audio range (> 1 Hz) are generated

by thunderstorms worldwide.

The AMT and MT geophysical methods

combine measurements made of site-

specific electric and magnetic fields

using grounded dipoles and magnetic

field antennas over a wide band of

frequencies. Low frequencies sample

deep into the earth and high frequencies

(AMT) correspond to shallow samples.

Ground resistivity values are calculated

from the magnitude and ratios of these

components and then mapped using

Zonge inversion and modeling software.

Ground resistivity relates to the geology.

Advantages of AMT

• No need for power source or high-

voltage electrodes.

• Minimal environmental impact.

• Stations can be acquired almost

anywhere and can be placed any

distance apart.

• Zonge’s backpack portable system

allows for use in difficult terrain,

• Relatively easy field logistics for

large-scale regional reconnaissance

exploration and for detailed surveys of

local geology. Fast data collection.

AMT Exploration Applications

• Mineral, groundwater and geothermal

exploration

• Excellent for imaging moderately

deep geologic structure and near-

surface geology to depths of about

500 meters in detail

• Onshore oil and gas exploration

where seismic is not feasible or is

cost prohibitive

• Mining and water resource

management

AMT is a passive

electromagnetic

imaging method

using the earth’s

magnetotelluric

field to map

geologic contacts

and structure.

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Field Logistics

The Zonge AMT equipment system

consists of the GDP multichannel data

processor (receiver), ANT/6 magnetic-

field sensor(s), electric-field cabling, and

non-polarizing electrodes for measuring

ground potentials. This system is easily

transported and multiple systems can be

synchronized to provide a remote

reference for noise cancellation.

AMT signals are extremely subtle and

measuring them requires highly sensitive

magnetometers, very low-noise electric-

field sensors, and careful installation.

The Zonge ANT/6 magnetic-field sensor

has a noise threshold level below typical

natural-source AMT signal levels. With

this system, it is possible to collect the

most useful data in the attenuation band

under the most conditions.

At each survey station, electrode pairs

are used to measure the electric (E)

field, and magnetometers are used to

measure the magnetic (H) field. The

electrical conductivity of the subsurface

affects the amplitude, phase, and

directional relationships of the E and H

fields on the earth’s surface. The

frequency ranges of interest are typically

from 4 to 8192 Hz for AMT.

“Ex” and “Ey” are used to designate

measured components of the electric

field. “Hx” and “Hy”

designate measured

components of the

magnetic field.

Different array types

may be used to

acquire the most

beneficial data for a

particular survey area

and objectives.

Scalar AMT is collected by a series of

electrode pairs, each measuring Ex, set

up along a survey line with one

magnetic-field sensor measuring Hy.

Tensor AMT acquires additional electric-

field Ey and magnetic-field Hx readings,

which provide information about

impedance directionality at a particular

location. The Hz component used on

lower-frequency MT surveys is not

usually recorded for AMT surveys.

Scalar AMT

• Easily modified to accommodate

terrain or other needs in the field

• Fast linear collection rates

• Limited in resolving or delineating

multi-dimensional targets

• Images the most geologic contacts

when data is collected roughly

perpendicular to geologic strike (if

known)

Tensor AMT

• Resolves ambiguity of strike which

may not be known at the beginning of

the survey

• Provides structural definition and

directionality

• Slower production rates compared to

Scalar array but still far faster than MT

Zonge’s backpack portable AMT acquisition system allows

for use in difficult terrain and relatively easy field logistics.

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Inversion and Modeling

Both 1D and 2D Smooth-Model inversion

software is used to convert measured E

and H components to resistivity versus

depth profiles. Smooth-model inversion

mathematically back-calculates (“inverts”)

from the measured data to determine a

likely location, size and depth of the

source or sources of resistivity changes.

Imaging natural-source AMT data is a

multistage process in which both

resistivity and phase are used in the

inversion to image resistivity changes

associated with the geology. Cagniard

Resistivity and Impedance Phase data

are calculated from the collected electric

field and magnetic field data. Cagniard

Resistivity is a frequency-dependent,

apparent resistivity calculation based on

the ratio of the E and H magnitude

components of the electromagnetic field.

Impedance Phase is defined as the

difference between the E and H phase

components.

Inversion models of AMT Cagniard

Resistivity and Impedance Phase data

provide detailed images of the

conductivity structure at depth.

The Zonge AMT modeling software

allows various parameters to be used to

create the most geologically reasonable

model. 1D and 2D software will produce

modeled depth sections for scalar AMT.

2D software will produce modeled depth

sections for tensor AMT.

2D inversion produces two-dimensional

shaped features (e.g., edges associated

with contacts at depth) and is able to

model any dipole orientation, but in

doing so assumes the survey line

crosses the geologic strike on the

perpendicular.

Lateral and Vertical Resolution

Inversion models show reasonable detail

to certain depths. The skin depth (depth

where field strength decreases to 37% in

a homogeneous earth) is considered the

depth of penetration. It is possible for

AMT results to image the subsurface at

greater depth in more resistive ground

by decreasing the floor frequency.

However there are practical limitations

related to size with resolving features at

depth. Survey resolution is also a

function of target size and conductivity

contrasts. Lateral survey resolution is

estimated at 15% of the depth of

investigation. This is one reason that

survey design for natural source AMT

often differs radically from that used for

MT surveys.

Zonge 2D smooth-model inversion corrects data for topographic effects. When possible,

AMT survey lines should be aligned perpendicular to geologic strike for best results.

Final Product

The results of processing and modeling

natural-source AMT data can be

presented in several forms: modeled

cross sections, plan views, fence or 3D

diagrams. When stations are collected

along several lines in the same area,

data can be displayed in plan-view plots

at a constant elevation or depth. Plan

views can help highlight trends between

lines. Fence diagrams show 2D cross

sections of the resistivity inversion

results in a spatially relevant 3D context.

Dotted lines in the fence diagram below

represent mapped faults.

Reference

Cagniard, L., “Basic Theory of the magnetotelluric method of geophysical

prospecting.” Geophysics, 18, pp. 605-635, 1953.

Goldstein, M.A. and Strangway, D.W., “Audio-frequency magnetotellurics with a

grounded electric dipole source.” Geophysics, 40, pp. 669-683, 1975.

Zonge, K.L. and Hughes, L.J., “Controlled source audio-frequency magnetotellurics.”

Electromagnetic Methods in Applied Geophysics, Vol. 2, edited by Nabighian, M.N.,

pp. 713-809. Society of Exploration Geophysicists, 1991.

Wight, D. E. and F. X. Bostick, “Cascade decimation, a technique for real time

estimation of power spectra.” Proceedings of IEEE Internal Conference on Acoustic,

Speech Signal Processing, Denver, Colorado, April, 1980.

Vozoff, K., “The Magnetotelluric Method.” Electromagnetic Methods in Applied

Geophysics, Vol. 2, edited by Nabighian, M.N., pp. 641-711. Society of Exploration

Geophysicists, 1991.

For more information, see www.zonge.com/geophysical-methods/

2015-02

Zonge International is

an employee-owned

company providing

ground geophysics field

services, consulting and

customized equipment

to geoscientists and

engineers worldwide.

The company is known

for its expertise in the

development and

application of broad-

band electrical and EM

methods.

Tucson, Arizona, USA

1 520-327-5501

Reno, Nevada

1 775-355-7707

zonge@zonge.com

Plan views can help highlight trends

between lines of stations.

Smooth model inversion results.

Fence diagrams in 3D context.